Photovoltaic devices are becoming increasingly popular for providing renewable energy. FIG. 1 shows one example of a photovoltaic device 100, which can be formed by depositing sequential thin film layers on a substrate 110. The photovoltaic device 100 may include a TCO stack 170 formed over the substrate 110, semiconductor layers 180 formed over the TCO stack 170, a back contact 155 formed over semiconductor layers 180 and a back support 160 formed over the back contact 155.
Generally, the substrate 110 is the outermost layer of a completed photovoltaic device 100 and, in use, may be exposed to a variety of temperatures and forms of precipitation, such as rain, snow, sleet, and hail. The substrate 110 may also be the first layer that incident light encounters upon reaching the photovoltaic device 100. It is therefore desirable to select a material for the substrate 110 that is both durable and highly transparent. For these reasons, the substrate 110 may include, for example, borosilicate glass, soda lime glass, or float glass.
The TCO stack 170 may include a barrier layer 115 formed on the substrate 110 for preventing sodium diffusion from the substrate 110 into the photovoltaic device. The barrier layer 115 may be formed of, for example, silicon nitride, silicon oxide, aluminum-doped silicon oxide, boron-doped silicon nitride, phosphorus-doped silicon nitride, silicon oxide-nitride, or any combination or alloy thereof. The TCO stack 170 further includes a TCO layer 120 formed on the barrier layer 115. The TCO layer 120 functions as the first of two electrodes of the photovoltaic device 100 and may be formed of, for example, fluorine doped tin oxide, cadmium stannate, or cadmium tin oxide. In addition, the TCO stack 170 includes a buffer layer 125 formed on the TCO layer 120 to provide a smooth surface for semiconductor material deposition. The buffer layer 120 may be formed of, for example, tin oxide (e.g., a tin (IV) oxide), zinc tin oxide, zinc oxide, zinc oxysulfide, and zinc magnesium oxide. It is possible to omit one or both of the barrier layer 115 and buffer layer 120 in the TCO stack 170 if desired.
Back contact 155 functions as the second of the two electrodes and may be made of one or more highly conductive materials, for example, molybdenum, aluminum, copper, silver, gold, or any combination thereof, providing a low-resistance ohmic contact. TCO layer 120 and back contact 155 are used to transport photocurrent away from photovoltaic device 100. Back support 160, which may be glass, is formed over back contact 155 to protect, together with substrate 110, photovoltaic device 100 from external hazards.
The semiconductor layers 180 may include a semiconductor window layer 130, for example, a cadmium sulfide layer, and a semiconductor absorber layer 140, for example, a cadmium telluride layer. The semiconductor layers 180 may further optionally include a transition semiconductor layer 145, for example, a cadmium zinc telluride layer, and a semiconductor reflector layer 150, for example, a zinc telluride layer. The semiconductor window layer 130 allows the penetration of solar radiation to the semiconductor absorber layer 140 which then converts solar energy to electricity through the formation of minority electron carriers. Specifically, semiconductor materials, like any other solids, have an electronic band structure consisting of a valence band, a conduction band and a band gap separating them. When an electron in the valence band acquires enough energy to jump over the band gap and reach the conduction band, it can flow freely as current. Furthermore, it will also leave behind an electron hole in the valence band that can flow as freely as current. Carrier generation describes processes by which electrons gain energy and move from the valence band to the conduction band, producing two mobile carriers: an electron and a hole; while recombination describes processes by which a conduction band electron loses energy and re-occupies the energy state of an electron hole in the valence band. In a p-type semiconductor material like the semiconductor absorber layer 140, electrons are less abundant than holes, hence they are referred to as minority electron carriers whereas holes are referred to as majority carriers.
During the conversion of solar energy to electricity at the semiconductor absorber layer 140, some minority electron carriers penetrate through the absorber layer 140 and may recombine with hole carriers, causing power dissipation inside the photovoltaic device 100, thereby reducing power conversion efficiency. Accordingly, optional semiconductor reflector layer 150 can be deposited over the semiconductor absorber layer 140 to act as a barrier or reflector against the minority electron carrier diffusion. The reflector layer 150 is formed of a semiconductor material with electron affinity lower than that of the absorber layer 140, for example, zinc telluride (ZnTe), which forces electron carrier flow back toward the electron absorber layer 140, minimizing minority electron diffusion. This is described in U.S. Provisional Patent Application 61/547,924, entitled “Photovoltaic Device And Method Of Formation,” filed on Oct. 17, 2011, the disclosure of which is incorporated herein by reference.
Although optional semiconductor reflector layer 150 reduces power dissipation and increases power conversion efficiency in the photovoltaic device 100, lattice mismatch may occur between the semiconductor reflector layer 150 and the semiconductor absorber layer 140, which can partially negate this benefit. In general, semiconductor materials contain a lattice, or a periodic arrangement of atoms specific to a given material. Lattice mismatching refers to a situation wherein two materials featuring different lattice constants (a parameter defining the unit cell of a crystal lattice, that is, the length of an edge of the cell or an angle between edges) are brought together by deposition of one material on top of another. In general, lattice mismatch can cause film growth against the natural grain of the adjacent film, film cracking, and creation of point defects at the interface between the two materials featuring the different lattice constants.
To reduce the effects of lattice mismatch between the semiconductor absorber layer 140 and the semiconductor reflector layer 150, the optional semiconductor transition layer 145, formed of a combination of the semiconductor absorber material and the semiconductor reflector material, for example, a Cd(1-x)Zn(x)Te layer where 0<x<1, may be formed between the semiconductor absorber layer 140 and the semiconductor reflector layer 150. By virtue of its composition, the semiconductor transition layer 145 has a lattice constant between that of the semiconductor absorber layer 140 and the semiconductor reflector layer 150, which reduces lattice mismatch between the two layers and increases the electronic conversion efficiency of the photovoltaic device 100.
Forming the semiconductor reflector layer 150 and the semiconductor transition layer 145 in a photovoltaic device may be hindered by the intrinsic nature of the combined semiconductor materials that compose the two layers, for example, cadmium (Cd), zinc (Zn) and telluride (Te). Specifically, ZnTe and CdZnTe are highly resistive films, which can cause flaws to form in the semiconductor reflector layer 150 and/or the semiconductor transition layer 145 as they are formed over other semiconductor layers, for example, a cadmium telluride layer 140. Incorporating conductive dopant into the ZnTe and CdZnTe layers can increase the conductivity and decrease the resistivity of these layers, which reduces the flaws that may occur during formation and provides the electrical properties necessary for the layers to function in a photovoltaic device. Accordingly, a method and apparatus for incorporating conductive dopant into zinc telluride and cadmium zinc telluride thin film layers during formation of photovoltaic devices is desired.